Shut Up and Dig

Exactly what wildlife do we want overwintering in our beds?

Somehow or other, this time of year I always come across the olde leaves-in-borders debate:  To remove them now or wait until spring (or never, letting them decompose slowly)?  This year it started innocently enough with a blog post for a garden center about what parts of my garden gets its leaves removed now, and why.  I used the photo above to show two plants that could be ruined by being buried under wet leaves for too long, so I remove the leaves around them soon after they fall.  These are Lambs’ Ears and a creeping Sedum, but I’d do the same for any dry-loving plant.

In the lower part of that photo are perennials that don’t mind being covered with leaves, so I’ll wait until all the leaves are down and maybe early spring before sending them off to the city’s compost-making operation.

Next, aesthetics.  Above is a very prominent spot just outside my porch and living room window where the plants wouldn’t complain about leaves but I’d rather have the view below all winter.  Also, I remove leaves from hard surfaces I walk on just for safety’s sake.

Then there’s the reason for early leaf removal that I was taught by a Carol Allen, trusted organic gardening guru to many in the DC area.  She does it to remove diseased plant parts that may winter over, and spots for garden pests like mice and voles to winter over, too.   She gets the leaf removal and mulching of her beds done by the end of December.

The Pro-Insect Position

But then someone sent me an article that entomologist Doug Tallamy recently wrote on the subject.

Unfortunately, the biggest threat to over-wintering insects doesn’t come from the weather. It comes from us, as we strive to “clean up” our flowerbeds, lawns, and meadows. A little knowledge about how insects spend the winter can help us avoid killing them unintentionally.”

Thick leaf litter is ideal for overwintering. This is just one more reason to retain, on your property, as many of your fallen leaves as you can.

The easiest way to preserve over-wintering insect populations is to relax our neatnik standards whenever possible. Plant the herbaceous perennials that spend the winter as dead stalks in less public areas of your properties. Come to realize that these stalks are not quite as dead as they seem, and are the normal condition of these plants in nature, a condition that many insects take advantage of during the fall and winter.

Well, I’m fine with leaving dead perennial stalks standing until spring; it’s the leaf litter that I worry about harboring pests that I’ll have to deal with next year – like aphids, black spot, voles and mice.   I use no products in my garden – organic or otherwise – and garden clean-up helps avoid the need for any.   Around trees and shrubs I think leaf litter’s probably fine, though.

Name those Critters!

But I’m on a quest for more specifics.  Exactly what beneficial or pesky critters and diseases could be encouraged by a nice thick layer of leaf litter among perennials?  Inquiring gardeners want to know!  And if no definitive answers get posted in comments, what experts should I consult?

On the  Toxicity of Introduced Plants

And this quote from Tallamy startled me:

Plants from Asia or Europe are typically toxic to our local insect herbivores, but these same insects are well equipped to avoid the toxic defenses employed by local native plants. 

Does toxicity here mean that insects are harmed by these plants?  Again, inquiring gardeners…. 

UPDATE: TALLAMY RESPONDS

In an email to me, which I also posted in the comment section:  Very helpful~!

Hi Susan,

First an answer about my “Toxic” statement. What I meant when I said nonnative plants are toxic to native insects was this: all plants defend their leaves from herbivores with secondary metabolic compounds, a cocktail of bitter or toxic chemicals that make the leaves taste bad. Over the eons, native insects have adapted to the defensive chemicals of one or two native plant lineages. They develop the enzymes and physiological capabilities that allow them to circumvent those plant lineages (think monarchs on milkweeds). In other words, 90% of our native insect herbivores have become specialists at eating what they have adapted to. But they cannot eat what they have not adapted to. Plants from Asia have unique defensive chemistry that our native insects have no adaptations for. In most cases, our insects could not eat Asian plants like Autumn olive, privet, or crape myrtle without dying. But our insects locate the host plants they have adapted to by smell. They smell the chemical signature of their host plant and they don’t make many mistakes. So our insects wouldn’t even try to eat nonnative plants because they don’t smell right. The big problem with nonnatives is that they are replacing the native plant communities that our insects depend on, both in our gardens and in nature, where nonnative invasive species (85% of which have come from our gardens) now comprise about 30% of the vegetation. Of course, most gardeners wonder why we want any insects. Just ask the birds you like to see in your yard. 96% of our birds rear their young on insects (not seeds and berries). No insects, no baby birds. No baby birds, no big birds.
As for leaves in the garden, just use common sense. In manicured gardens close to the house, you may not want a thick bed of leaf litter, as good as it is for your soil. You are right that leaves give mice and voles good cover (although aphids do not over winter in leaves). Insects that spend the winter as eggs or pupae usually rely on brush for cover, though some will hide in leaves. The biggest benefit of leaves is that they are the perfect mulch for our soil and our trees. When we force grass under trees, it shortens their lives by decades.
Hope this helps
Doug

Posted by on November 16, 2012 at 7:50 am, in the category Shut Up and Dig.
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29 Responses to “Exactly what wildlife do we want overwintering in our beds?”

  1. Karla says:

    Great post & great suggestions, especially about voles, and leaves being slippery.

    I’ve left leaves in my gardens for at least 10 years–perhaps longer. I too am organic and don’t use any products, organic or otherwise. I haven’t noticed an increase in “bad” bugs at all but I have noticed an increase in beneficials, butterflies and earthworms.

    One place I have started removing the leaves is a bed with rudbeckia. I’ve got a leaf beetle that damages the leaves of those plants and I’d prefer it not winter in the litter there.

    Hope that helps!

  2. Although I admit I never check during the cold of winter, my own experience with leaf litter at other times of the year says that you’ll find lots of spiders as well as moth and butterfly cocoons and pupae. I’d expect that ladybeetles may find refuge there too, and possibly lacewings.

    I think that if you have ornamental grasses that you leave in place through the winter then the need for leaf litter on the ground is decreased. I know my firewood stacks always end up being winter homes for lots of different creatures too (and I feel guilty about disturbing them).

  3. Lisa-St. Marys ON says:

    I stole my neighbours leaves and added them to my beds last year. I certainly did not see an increase in bad bugs. I have seen more worms, and more butterflies, oddly not as many caterpillars.

  4. Frank Hyman says:

    Boy someone needs to tell Mother Nature that she needs to clean up all those leaves out in the woods–they’re harboring horrible, scary diseases and bugs that will surely defoliate the planet and send hordes of vermin into our homes.

    Okay, sorry for the snark. But that’s the kind of reaction I have to these “old gardener tales” that don’t stand up to a few seconds of scrutiny along the lines of “how does this work out in nature?”

    I suspect that lot of the beliefs out there, such as this one, have more to do with a character trait that is a word that starts with an “a” and rhymes with “anal.” Okay it doesn’t Just rhyme.

    Like another poster, I gather up the bags of leaves that other homeowners laboriously deposit on the curb for me (one day I swear I will leave a “thank you” note), dump them in my sideyard, shred them with the elec. mower and spread them as mulch between Christmas and Valentines day. Why then? B/c it’s between the period after some deciduous plants get cut back and before most bulbs start coming up, so there’s plenty of room for mulching. And we have plenty of beneficial insects of all stripes.

    Basic good gardening practices: right plant/right spot, annual mulching, not too much nitrogen, etc. are the best bets for beating 97% of pest problems. Nature is not much concerned about tidiness or perfection. Good luck!

  5. My preference is to keep all leaves on site through the winter, moving them off walkways and other hard surfaces. I find it perplexing that people will take the leaves out of their garden, send them off for greencycling, then buy compost. If you have particularly large leaves, it’s best to shred them first so they don’t make a wet anaerobic mat. That can cause some disease problems. Other than that, there really aren’t any drawbacks. Any toxins in leaves – native or otherwise – are broken down by microbes. Rodents aren’t particularly attracted to decomposing leaves, but they do love burrowing under sheet mulches like cardboard or geotextiles. And the aesthetic issue can be taken care of very easily by covering the leaves with your preferred mulch – in my case, arborist wood chips. It keeps the leaves in place, where their nutrients can be cycled back into the system.

  6. Shannon says:

    Funny. I had an argument with my leaf-blower-happy father about not cutting back his ornamental grasses (I wouldn’t dream of telling him not to clean up leaf litter. It’s his passion). He ended up destroying a critter’s nest one day when I wasn’t there to stop him.

  7. Susan Harris says:

    Great comments with the kind of info gardeners can really use. Thanks and keep ‘em coming.

  8. Great information and questions, Susan! Selective clean-up is definitely the right approach for balancing human and ecological needs in the winter garden. Remove debris around plants prone to pests and disease — irises, garden phlox, roses, edibles — while leaving it around that plants that will tolerate or benefit from the extra mulch.

    In answer to your question about specific benefits: While most insects overwinter in stems, ladybugs, centipedes, and some varieties of native bees overwinter or lay eggs in leaf litter. Birds will pick through dead plant debris when food is scarce in winter, and in spring it becomes an important source of nesting material.

  9. Deirdre says:

    Back when I was smothering grass for larger beds, I’d steal bagged leaves from my neighbors’ curbs. I don’t need to do that anymore. I’m glad. It felt a little strange. Of course, I did it anyway.

    I rake or mow leaves that fall on the grass, deck, patio, and walkways. I dump them in the beds; whole leaves where I want to smother weeds, shredded where I want to feed the lilies, etc. In hort school I learned that 80% of a tree’s nutritional needs are met by it’s own fallen leaves. Why would I want to starve my trees? When they, the leaves not the trees, have all fallen, I selectively remove them from shrubs and groundcovers they might harm. The only pests I’ve noticed are the #@*& slugs, but I’d have them anyway. The juncos and towhees spend the winter ruffling through the leaves looking for bugs.

    Why pay for mulch when it falls out of the sky?

  10. tropaeolum says:

    Susan., did you email Professor Tallamy asking him to respond to your post? It seems like he would be happy to do so.

  11. Peter says:

    Since most of my yard right now is grass, I still rake up my leaves and compost them. I will say though that, since I bought a new lawnmower this year that has a bag (the old one I had, from the previous homeowner, did not have one) the task is a lot easier plus getting the grass clippings into the mix is a boost.

    The one thing I have found the most immediate use for leaves for is to mulch up the garlic plantings. While I do then take some off early in the spring I leave a lot of it on to keep the weeds down.

  12. Paul W says:

    I can’t comment on mice or insects because I’ve not paid much attention to either in and under the leaves in my garden beds, but I can agree wholeheartedly with Susan that voles can be a major issue if they’re protected by a nice layer of leaves.

    We have a booming vole population in our rural VA garden. I’m not poisoning them or even trying to trap them because I’d rather spend that time on other garden projects. We also don’t cut back many of our perennials or grasses until late Feb or early March because we love the look of all the seed heads in winter – it’s a study in brown :) When we do finally clean up our beds in spring, and rake leaves out of the areas where they’ve overwintered, there is quite a lot of evidence of voles munching on plants from their protected position. Between our cat, a relatively high number of hawks, resident foxes AND coyotes, we should have plenty of vole predators, but a nice cover of leaves is apparently protection enough.

  13. Chris says:

    I’m torn every year between leaving the Norway maple leaves where they fall under the tree into a large bed of big leaved aster and woodland sunflower, or raking them all up because they, like the leaves on every Norway maple on my street, are covered with black tar spot.

    I doubt it would be a Good Thing to rake and spread them, shredded or not, into the rest of the garden.

    As it is, in order to rake em all up I’d have to cut back the perennial stalks close to the ground and have near bare dirt over winter. Is it worth it to try and control the black spot?

    • gemma says:

      Cornell says mulching can destroy the fungus, but the mulch should be covered or turned before new leaves emerge in the spring. http://plantclinic.cornell.edu/factsheets/tarspotofmaple.pdf

      Since it’s on every tree on your street, it’s not going to go away. I’d do whatever takes the least amount of work. If it were my tree, I’d probably add a good layer of mulch in the spring well before the leaves emerge.

  14. gemma says:

    One of the most important things I learned in hort classes and from both books and experience: the number-one thing that can improve *any* garden is adding mulch. So I’d rake the leaves off the hardscape, but keep the other leaves in place and cover them with more mulch if they’re blowing around.

    Gardens with large areas of bare soil look unfinished to me, and usually the plants are struggling, too. In California some native bees live in the ground, so it’s a good idea to leave a small bare patch for them, but otherwise, mulch mulch mulch.

  15. Nora (Seattle) says:

    I try and leave most leaves in my garden, but there are a few exceptions:

    1) My roses continually struggle with black-spot, and I clean their leaves up right away.

    2) My neighbor’s 50′+ mountain ash drops not only leaves but berries around the same time. In order to control the mountain ash volunteers (which arrive by the thousands), I actually take my husband’s shop-vac & clean them up.

    Last year, I rescued my rosmary from the first hard frost by throwing a big pile of leaves on them. It did the trick! All but one survived, while my neighbor lost 5 plants.
    I have a lot of ground-covers that i leave to their own devices & hope they survive being blanketed over winter. I am curious what other’s do? I am talking sedums, pratia, scotch moss, brass buttons and ajuga.

  16. Susan, please paint that fence outside your porch and living room window. It is making me crazy.

    As for raking leaves out of the beds it depends on what is in the bed, what the client wants and where I can put them. In this part of the world the city sends out a big truck that vacuums leaves from the curb. I keep at least 90% on site because I can’t bear the thought of tossing free soil conditioner away even if it won’t be used for millenia because it doesn’t end up in an actual part of the garden.

  17. Laura says:

    I don’t have voles so they aren’t a concern for me here.

    The local organic gardening radio show guru tells us that leaves are nature’s way for trees to replenish their own nutrients, and he suggest leaving them in place.

    Regardless of whether or not he’s right, I rarely rake up my oak leaves and I actually add leaves from a landscaper friend’s bags to all of my beds, which act a cheap mulch. So far, no problems.

    I’ve also used leaves as insulation on potted plants when we had really bad freezes. They worked.

  18. Rachel says:

    With respect to roses, in warm climate disease prone areas like mine, I would definitely remove all rose foliage after pruning and defoliating. All other leaves (except citrus) are vital parts of the mulch layer which protects our dry climate from evapotranspiration and adds organic material to the soil.

    I find that a leaf mulch provides habitat for springtails, centipedes, all sorts of beetles, slender salamanders, and of course my favorites, mushrooms including blewitts, shaggy parasols, and assorted less appetizing but beautiful fungi.

  19. Anne Wareham says:

    I love this – someone asking questions while all around everyone tells us what to do. (on what basis? I ask myself – there are not many truly experimental gardeners)

    It must be very different here in UK. Never seem to need to worry about pests or using pesticides apart from slugs, and a leaf regime is not going to impact there.

    Have no idea what good and bad things are hiding under my leaves but all things seem to thrive if I just let it all dessicate over winter with the leaves, wherever they have landed, and jump on the dead bits in spring. Which is quite fun and satisfying.

    Is it possible that the natural world is a damn sight more resilient that garden-writers-needing-things-to-write-about like to pretend?

  20. Kate Kruesi says:

    I think we need to work WITH the ecology of our garden’s site as much as possible and continuously question “how nature does it”, i.e. get a garden site into a sustainable equilibrium without adding annual inputs, e.g. mulch (or organic/”inorganic” fertilizer, or even water, for that matter). This involves, as Frank Hyman said, right plant in the right place (re: insect/fungal disease/site tolerance), and the ongoing garden experimentation to which Anne Wareham alluded.

    That being said, I’m not sure one would have much of a garden in a suburban oak woodland! Which begs the question, what tree/plant diversity does one find in an intact regional oak woodland?

    Susan, the checkerboard garden looks great – awesome work!! If I were gardening it, aesthetically and ecologically, I would rake the leaves off the checkerboard to underneath the hydrangeas and around the edges, achieving a “windswept” look, i.e. a little whirlwind (it happens around corners!) blew the leaves off most of the checkerboard to under the “taller leaf and snow collecting” plants around the edges if you get my metaphor. Also, it would be instant “leaf management/composting in place”!

    Great post, great discussion!

  21. Kate Kruesi says:

    Oops, I forgot to add re: wildlife, that vole populations, etc. are cyclical (7 or 9 year cycle?). We can’t “control it all”. We can only garden “around/with reality”.

    We need to make space for the predators, too, mentioned by other commenters above. And remember, to keep said predator population viable, there needs to be enough ongoing prey to consume, i.e. we’ll have to live with a certain number of voles in our garden!

  22. Gail says:

    Despite the best efforts of my cats I do have a vole problem. In the next week or two I cut down and remove to my large composting area all of my ornamental grasses in one of my beds. I am more selective in other beds. I do remove the peony leaves and hosta leaves. I do have beds that the leaves naturally blow in and I leave those through the winter. We mow and mulch all our leaves and I admit I’m a leaf thief too. In fact I visit the local yard waste center and take all those hay/straw bales that people toss out this time of year for mulch for my veg garden next year.

  23. Susan Harris says:

    Doug Tallamy responded to me in this helpful email:

    Hi Susan,
    First an answer about my “Toxic” statement. What I meant when I said nonnative plants are toxic to native insects was this: all plants defend their leaves from herbivores with secondary metabolic compounds, a cocktail of bitter or toxic chemicals that make the leaves taste bad. Over the eons, native insects have adapted to the defensive chemicals of one or two native plant lineages. They develop the enzymes and physiological capabilities that allow them to circumvent those plant lineages (think monarchs on milkweeds). In other words, 90% of our native insect herbivores have become specialists at eating what they have adapted to. But they cannot eat what they have not adapted to. Plants from Asia have unique defensive chemistry that our native insects have no adaptations for. In most cases, our insects could not eat Asian plants like Autumn olive, privet, or crape myrtle without dying. But our insects locate the host plants they have adapted to by smell. They smell the chemical signature of their host plant and they don’t make many mistakes. So our insects wouldn’t even try to eat nonnative plants because they don’t smell right. The big problem with nonnatives is that they are replacing the native plant communities that our insects depend on, both in our gardens and in nature, where nonnative invasive species (85% of which have come from our gardens) now comprise about 30% of the vegetation. Of course, most gardeners wonder why we want any insects. Just ask the birds you like to see in your yard. 96% of our birds rear their young on insects (not seeds and berries). No insects, no baby birds. No baby birds, no big birds.

    As for leaves in the garden, just use common sense. In manicured gardens close to the house, you may not want a thick bed of leaf litter, as good as it is for your soil. You are right that leaves give mice and voles good cover (although aphids do not over winter in leaves). Insects that spend the winter as eggs or pupae usually rely on brush for cover, though some will hide in leaves. The biggest benefit of leaves is that they are the perfect mulch for our soil and our trees. When we force grass under trees, it shortens their lives by decades.

    Hope this helps
    Doug

  24. UrsulaV says:

    For specific good bugs, let me mention a couple critters that overwinter in leaf litter–luna moths and hummingbird clearwing moths (both as cocoons in leaf litter), bumblebee queens (most bumblebees die off in winter, but the queens overwinter under bark or leaves) and mourning cloaks (the last group of adults overwinter under tree bark or in leaf cover, and emerge very early in spring.)

    Plus sometimes you’ll find gray tree frogs hibernating under leaf cover.

    If the leaves are just moved to a border and dumped, the damage to most of them is probably minimal–I expect the bumblebees might suffer a bit in the cold, but that’s about it. It’s the bagging and shredding that kills so many moths. Luna moth populations on in a steep decline, probably owing to just that thing.

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